The present invention relates to the field of fluorohydrocarbons and its subject is more particularly the manufacture of difluoromethane (F32) by catalytic fluorination of methylene chloride.
Difluoromethane, known under the designation F32, is harmless to the ozone layer. It is therefore particularly advantageous for replacing CFCs. As a mixture with other hydrofluoroalkanes such as 1,1,1-trifluoroethane (F143a), 1,1,1,2-tetrafluoroethane (F134a) or pentafluoroethane (F125), it is intended especially to replace F22 (chlorodifluoromethane) and F502 (azeotropic mixture of F22 and of chloropentafluoroethane) in the field of refrigeration, of conditioned air and in other applications.
There are various known processes for the synthesis of F32. The hydrogenolysis of F12 (dichlorodifluoromethane) or of F22 (patents JP 60-01731 and EP 508 660) has the disadvantage of being generally not very selective and of producing worthless methane as by-product. It has recently been proposed to produce F32 by fluorination of bis(fluoromethyl) ether (patent EP 518 506).
It is also possible to produce F32 by fluorination of methylene chloride (F30) with anhydrous HF. Many patents describe this reaction, claiming the use of catalysts such as Cr2O3, CrF3, AlF3, Cr/carbon, Ni/AlF3 etc.
The difficulty of this reaction lies in the stability of the catalyst, which tends either to coke rapidly or to crystallize. The problem becomes very tricky if it is intended to combine a high space time yield and a good selectivity while maintaining good stability of the catalyst.
To reduce this deactivation it has been proposed to employ specific catalysts such as a mechanical mixture of alumina and chromium oxide (patent GB 821 211). This patent gives an example for the fluorination of methylene chloride but the F32 space time yields obtained on this catalyst are low ( less than 200 g/h/l) and the cumulative duration of the tests is shorter than 5 hours.
More generally, during fluorination reactions, it is very often envisaged to inject oxygen or air continuously in order to lengthen the lifetime of the catalysts. Thus, patent JP 51-82206 claims the use of 0.001 to 1% of oxygen to maintain the activity of catalysts prepared from chromium oxide. The examples in this patent relate only to reactions of fluorination of perhalogenated molecules (CCl4, C2Cl3F3).
The major disadvantage of this process is the appearance of a Deacon reaction. In fact, chromium oxide, well known as a fluorination catalyst, is also a good catalyst for the oxidation of HCl (patents U.S. Pat. No. 4,803,065 and U.S. Pat. No. 4,822,589). The oxygen introduced during the fluorination reaction reacts with the HCl formed to produce water and chlorine. Because of corrosion problems, the presence of water is particularly undesirable in a fluorination process.
Continuous introduction of a small quantity of chlorine has already been proposed in patent JP 49-134612, in order to stabilize the activity of the catalysts employed for the disproportionation of perhalogenated molecules; in this case the use of chlorine does not result in a decrease in the selectivity.
More recently the use of chlorine as an inhibitor of deactivation has also been described in the case of the fluorination of CF3CH2Cl (US Statutory Invention Registration H1129). The examples which are presented clearly show that the use of chlorine makes it possible to maintain a stable space time yield of CF3CH2F (F134a). On the other hand, no indication is given as to the effect of chlorine on the selectivity of the reaction.
However, in the case of hydrogenated molecules and in particular in the case of the fluorination of CF3CH2Cl (F133a), the presence of chlorination reactions has been demonstrated, resulting in the formation of worthless by-products. Thus, in the case of the fluorination of F133a, by-products of the F120 series (C2HClnF5-n) are chiefly formed.
Given that the Deacon reaction produces chlorine, this loss in selectivity is also observed during the fluorination of hydrogenated molecules in the presence of oxygen on chromium catalysts. This is why some patents (see, for example, patent EP 545 883) have claimed the preparation of specific catalysts limiting the oxidation of HCl and the by-production of chlorine.
It could be assumed that the behaviour of methylene chloride would be similar to that of F133a, and this would make the use of chlorine not very advantageous for maintaining the activity of the catalyst. However, it has been surprising to find that, in the case of the methylene chloride fluorination, even with relatively high contents (Cl2/F30=3 mol %), chlorine undergoes very little reaction with the compounds of the F30 series (CH2ClnF2-n), and this allows it to be employed without any significant decrease in the selectivity of the reaction.
In addition, in the abovementioned patent JP 51-82206 it is indicated that oxygen enables the catalyst activity to be maintained even in concentrations that are lower than that employed with chlorine. However, it has been found that, during the fluorination of methylene chloride, the continuous introduction of chlorine is, at an equal concentration, a more effective means than the addition of oxygen in order to stabilize the activity of the catalysts. In fact, in high space time yield conditions, oxygen addition is not sufficient to maintain the activity of the catalysts even at high temperature, whereas the addition of chlorine enables their lifetime to be significantly lengthened from a temperature of 250xc2x0 C. upwards and therefore the fluorination of methylene chloride to be carried out in a temperature range in which an irreversible deactivation of the catalyst by crystallization is not very probable.
The subject of the present invention is therefore a process for the manufacture of difluoromethane by gas-phase catalytic fluorination of methylene chloride by means of anhydrous HF, characterized in that the operation is carried out in the presence of chlorine.
In accordance with the process according to the invention, chlorine (pure or diluted in an inert gas such as nitrogen or helium) is introduced into the reactor at the same time as methylene chloride and HF.
The Cl2/CH2Cl2 molar ratio may vary within wide limits and is generally between 0.01% and 10%. A Cl2/CH2Cl2 molar ratio of between 0.05 and 5% is preferably employed and, more particularly, a molar ratio of between 0.1% and 3%. It is also possible to introduce chlorine by dissolving it in methylene chloride.
The reaction temperature is generally between 200 and 450xc2x0 C. However, the operation is preferably carried out at a temperature of between 250 and 380xc2x0 C. in order to obtain a high space time yield without risking a deactivation of the catalyst due to crystallization.
The fluorination catalysts to be employed for making use of the process according to the invention may be bulk catalysts or supported catalysts, the support which is stable in the reaction mixture being, for example, an active carbon, an alumina, a partially fluorinated alumina, aluminium trifluoride or aluminium phosphate. Partially fluorinated alumina is intended to mean a composition which is rich in fluorine and contains chiefly aluminium, fluorine and oxygen in proportions such that the quantity of fluorine expressed as AlF3 constitutes at least 50% of the total weight.
Among the bulk catalysts it is possible to mention more particularly chromium(III) oxide prepared according to any one of the methods known to a person skilled in the art (sol-gel process, precipitation of the hydroxide from chromium salts, reduction of chromic anhydride, and the like) and chromium trifluoride. Derivatives of metals such as nickel, iron, vanadium (in the oxidation state III), manganese, cobalt or zinc may also be suitable by themselves or in combination with chromium, in the form of bulk catalysts, as well as in the form of supported catalysts. Alkaline-earth metals, rare earths, graphite or alumina can also be incorporated in these catalysts or in their support in order to increase the thermal or mechanical stability thereof. During the preparation of catalysts using a number of metal derivatives in combination, the catalysts may be obtained by mechanical mixing or by any other technique, such as coprecipitation or a coimpregnation.
The supported or bulk catalysts can be employed in the form of beads, extrudates, tablets, or even, if operating in a stationary bed, in the form of fragments. When the operation is carried out in a fluid bed it is preferred to employ a catalyst in the form of beads or extrudates.
As nonlimiting examples of catalysts there may be mentioned:
chromium oxide microbeads obtained by the sol-gel process as described in patent FR 2 501 062,
catalysts with chromium oxide deposited on active carbon (patent U.S. Pat. No. 4,474,895), on aluminium phosphate (patent EP 55 958) or on aluminium fluoride (U.S. Pat. Nos. 4,579,974 and 4,579,976),
mixed chromium oxide and nickel chloride catalysts deposited on aluminium fluoride (patent application EP 0 486 333),
bulk catalysts based on crystallized chromium oxide (patent application (EP 657 408),
bulk catalysts based on nickel and chromium oxide (patent application EP 0 546 883),
bulk catalysts based on vanadium and chromium oxide (patent application EP 0 657 409).
The abovementioned patents, the content of which is incorporated here by reference, describe broadly the method of preparation of these catalysts, as well as their method of activation, that is to say of preliminary conversion of the catalyst into stable active species by fluorination by means of gaseous HF diluted with compounds which are inert (nitrogen) or not (air or 1,1,2-trichloro-1,2,2-trifluoroethane). During this activation the metal oxides serving as active material (for example chromium oxide) or as support (for example alumina) may be partially or completely converted to corresponding fluorides.
Mixed catalysts based on chromium and nickel, which are described in patent applications EP 0 486 333 and EP 0 546 883 are more particularly preferred.
The contact time, defined as the ratio of the total flow rate of the reactants (measured in the reaction conditions) to the volume of catalyst, may vary within wide limits and is generally between 0.01 and 20 seconds. In practice it is preferable to work with contact times of between 0.1 and 5 seconds.
This reaction may be carried out at atmospheric pressure or at a higher pressure. A pressure of between 1 and 20 bar absolute is preferably chosen.
The following examples illustrate the invention without limiting it.